The Senegal international has joined Christian Gourcuff's side on a four-year contract, saying the opportunity to move to Camp Nou came too early in his career.
The transfer comes just a year after Sarr's arrival in Metz, but he said he was looking forward to developing his game under Gourcuff.
Sarr, who was named as one of the 50 best Under-19 talents in the world, told the club's official website: "It's my pleasure to sign for Rennes. The adventure at Rennes will soon begin for me, and I already feel red and black.
"I left Senegal a year ago to join Metz, and I left my family there. I had to adapt to a new environment.
"Today I am happy, my parents are proud of me. I am grateful for every day that I get the chance to make my passion my profession."
Rennes faced competition from a number of clubs hoping to sign Sarr, including last season's La Liga runners-up Barcelona.
Sarr explained his decision to stay in Ligue 1, saying: "Rennes is the club that can help me to progress. I still have to develop defensively and become more decisive in my approach to the goal. With Christian Gourcuff, I am convinced that I will succeed.
"I resumed training with Metz thinking I would stay there and continue to progress. I had the opportunity to join Rennes and I am very proud to have done so.
"For me, leaving Metz to join Barcelona was too early."